Remains of _Geomys_ are abundant, especially from Pleistocene deposits of the Great Plains, but in most instances specific assignment is difficult or impossible since only isolated teeth or fragments of skulls have been preserved. Estimates of phyletic relationships of the known species of _Geomys_ are depicted in Figure 8; those estimates are useful in discussing the phyletic development of the genus. One of the earliest known species, _Geomys quinni_, ranges from Upper Pliocene to the later stages of the Lower Pleistocene (Aftonian interglacial deposits). The dentition of _G. quinni_ is essentially the same as in the living species except that open lateral re-entrant angles are retained in the premolars. _Geomys paenebursarius_, also of the early Pleistocene, is a smaller species and seems to be more directly in the line of evolution of the modern species. As yet unnamed smaller species of _Geomys_ from the Rexroad fauna (late Pliocene) and Saunders fauna (latest Aftonian) may also be on the main line of evolution. Surprisingly, _Geomys tobinensis_ and _Geomys garbanii_ of later Irvingtonian provincial age are less specialized than either _Geomys quinni_ or _Geomys paenebursarius_. It is likely that _G. tobinensis_ and the unnamed species from the Dixon are closer to the main line of descent than _G. paenebursarius_ suggesting that the direct ancestral lineage of the living species of _Geomys_ was more conservative and less specialized than _Geomys paenebursarius_ of the Lower Pleistocene. _Geomys quinni_ and _G. paenebursarius_ seem to have acquired specialized dental features in the early Pleistocene. _Geomys quinni_ was successful on the Great Plains, and persisted into the late Blancan. The main line may be represented in the early Pleistocene by _Geomys paenebursarius_ from the Hancock formation of the Texas Trans-Pecos. The structure of _G. paenebursarius_ indicates that it is in or close to the main line of descent, and probably evolved from one of the more primitive late Pliocene species of _Geomys_ from the Rexroad fauna. [Illustration: FIG. 8. Tentative arrangement of species of the genus _Geomys_, depicting phylogenetic trends and probable relationships within the genus.]